Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C23/00—Alloys based on magnesium
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
  • C01B3/0005—Reversible uptake of hydrogen by an appropriate medium, i.e. based on physical or chemical sorption phenomena or on reversible chemical reactions, e.g. for hydrogen storage purposes ; Reversible gettering of hydrogen; Reversible uptake of hydrogen by electrodes
  • C01B3/001—Reversible uptake of hydrogen by an appropriate medium, i.e. based on physical or chemical sorption phenomena or on reversible chemical reactions, e.g. for hydrogen storage purposes ; Reversible gettering of hydrogen; Reversible uptake of hydrogen by electrodes characterised by the uptaking medium; Treatment thereof
  • C01B3/0031—Intermetallic compounds; Metal alloys; Treatment thereof
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
  • C01B3/0005—Reversible uptake of hydrogen by an appropriate medium, i.e. based on physical or chemical sorption phenomena or on reversible chemical reactions, e.g. for hydrogen storage purposes ; Reversible gettering of hydrogen; Reversible uptake of hydrogen by electrodes
  • C01B3/001—Reversible uptake of hydrogen by an appropriate medium, i.e. based on physical or chemical sorption phenomena or on reversible chemical reactions, e.g. for hydrogen storage purposes ; Reversible gettering of hydrogen; Reversible uptake of hydrogen by electrodes characterised by the uptaking medium; Treatment thereof
  • C01B3/0031—Intermetallic compounds; Metal alloys; Treatment thereof
  • C01B3/0042—Intermetallic compounds; Metal alloys; Treatment thereof only containing magnesium and nickel; Treatment thereof
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C30/00—Alloys containing less than 50% by weight of each constituent
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/32—Hydrogen storage

Definitions

• This invention relates to hydrogen storage materials and particularly relates to a cast alloy which can be used as a hydrogen storage material.
• Hydrogen energy is attracting a great deal of interest and is expected to eventually be a replacement for petroleum based fuels.
• the main obstacle being the development of a viable hydrogen storage system. While hydrogen can be stored as a compressed gas or a liquid, the former occupies a large volume and the latter is energy intensive to produce, reducing any environmental benefits.
• both gaseous and liquid hydrogen are potentially dangerous should the pressure storage vessels be ruptured.
• a safer, more compact method of hydrogen storage is to store it within solid materials.
• metals and inter-metallic compounds can absorb large quantities of hydrogen in a safe, solid form.
• the stored hydrogen can be released when required by simply heating the alloy.
• Storage of hydrogen as a solid hydride can provide a greater weight percentage storage than compressed gas.
• a desirable hydrogen storage material must have a high storage capacity relative to the weight of the material, a suitable desorption temperature, good kinetics, good reversibility and be of a relatively low cost.
• Pure magnesium has sufficient theoretical hydrogen carrying capacity at 7.6 wt %. However the resulting hydride is too stable and the temperature must be increased to 278°C for the hydrogen to be released. This desorption temperature makes such materials economically unattractive. A lower desorption temperature is desirable to not only reduce the amount of energy required to release the hydrogen but to enable the efficient utilisation of exhaust heat from vehicles to release the hydrogen.
• the compound Mg 2 Ni has a reduced hydrogen storage capacity of 3.6 wt % but, importantly, the temperature required for hydrogen release is decreased to less than that of pure magnesium.
• the mechanism of hydrogen storage is believed to involve the formation of (solid) hydride particles, i.e. MgH 2 and Mg 2 NiH 4 in the microstructure.
• the invention may provide a method of producing a hydrogen storage material including the steps of forming a magnesium-nickel melt having additions of at least one refining element, the refining element being able to promote a refined eutectic structure with increased twinning in the magnesium-nickel intermetallic phase and solidifying the magnesium-nickel melt to a hydrogen storage material with said refined eutectic structure.
• the magnesium-nickel melt is formed by the steps of adding nickel to the magnesium melt to produce a hypoeutectic magnesium-nickel alloy within the range of greater than zero and up to 50 wt % Ni homogenising the magnesium-nickel melt, and adding the refining element or elements to the melt under a protective atmosphere at addition rates of greater than zero and up to 2 wt% and preferably greater than zero and less than 500 ppm.
• the refining element preferably has an atomic radius within the range of about 1-1.65 times that of magnesium. It is understood that refining elements with atomic radii within this range will provide the refined eutectic structure discussed above.
• the preferred refining elements are selected from the group comprising Zr, Na, K, Ba, Ca, Sr, La, Y, Yb, Rb, Cs and rare earth elements such as Eu. Zirconium is added to grain refine the magnesium crystals and when used requires at least one more of the elements from the list.
• the invention may provide a method of producing a hydrogen storage material comprising the steps of forming a magnesium nickel melt having additions of at least one refining element having an atomic radius within the range of 1-1.65 times that of magnesium, the refining element being provided in the melt at addition rates greater than zero and up to 2 wt% and preferably less than 500 ppm, and solidifying the magnesium nickel melt.
• the solidifying step in both aspects is preferably a casting step where the metal is cast by a suitable procedure such as pouring into preheated metallic moulds coding the casting.
• the solidifying step may be other controlled solidifying processes.
• the alloy is then subject to activation and use as a hydrogen storage material.
• the alloy is preferably used in the cast condition.
• a hydrogen storage alloy comprising or consisting essentially of greater than zero and up to 50 wt % Ni; greater than zero and up to 2 wt % of a refining element, the refining element having an atomic radius of about 1-1.65 times that of magnesium; and the balance magnesium and incidental impurities.
• the preferred refining additions are selected from the group of Zr, Na, K, Ba, Ca, Sr, La, Y, Yb, Rb, Cs and rare earth elements with addition rates greater than zero and up to 2 wt% and preferably greater than zero and less than 500 ppm.
• the more preferred addition elements are sodium and zirconium.
• the material is produced by a casting solidification process, it is a more commercially viable process for large scale mass production of hydrogen storage components.
• Fig 1 is a pressure composition temperature graph of an unmodified magnesium alloy with 14% Ni,
• Fig 2 is a graph summarising activation time at 350 0 C and 2 MPa for Examples 1-6,
• Fig 3 is a graph of PCT absorption data at 350 0 C and 2 MPa for Examples 1-6,
• Fig 4 is a graph of PCT absorption data at 300 0 C and 2 MPa for Examples 1-6,
• Fig 5 is a graph of PCT absorption data at 250 0 C and 2 MPa for Examples 1-6,
• Fig 6 is a graph summarising PCT absorption capacity at 350°C and 2 MPa
• Fig 7 is a graph illustrating the relationship between absorption and desorption for unmodified Mg 14 Ni alloy
• Fig 8 is a graph of the desorption data at 0.2 MPa for Examples 1-6.
• Fig 9(a)-9(h) are SEM micrographs of the as cast alloys of Examples 1-6.
• the hydrogen storage material is produced according to the invention by forming a magnesium-nickel by adding nickel to molten magnesium.
• the nickel addition may be up to 50 wt % nickel but the preferred addition rate provides an alloy melt of 10-20 wt % nickel.
• the melt is then mixed to provide a homogenised mix.
• magnesium-nickel alloy trace elements of crystallography modifying material are added.
• the elements added are those that refine the magnesium phase and promote a refined eutectic structure with increased twinning in the magnesium-nickel intermetallic phase.
• the range of elements satisfying the above two criteria have atomic radii around that of magnesium and up to 1.65 times that of magnesium and include Zr, K, Na, Ba, Ca, Sr, La, Y, Yb, Rb, Cs and rare earth metal elements.
• the preferred elements used are sodium and/or zirconium.
• the melt is again stirred to homogenise the mix and held under a protective atmosphere during the homogenising step.
• the protective atmosphere is any atmosphere which prevents the magnesium from combusting. Typical atmospheres include SF 6 and HFC- 134a.
• the metal is then cast by a suitable casting procedure such as by pouring into preheated metallic moulds.
• the hydrogen absorption of metal hydride alloys is characterised using equilibrium pressure composition temperature (PCT) data. This data is obtained by keeping an alloy sample at constant temperature while precisely measuring the quantity of hydrogen sorbed and the pressure at which sorption occurs.
• the quantity of hydrogen sorbed is expressed in terms of alloy composition, either as an atomic ratio of hydrogen atoms to the number of atoms in the base metal alloy or as the capacity of hydrogen in the material on a weight percent basis.
• PCT pressure-composition-isotherm
• the activation time (At) of the alloy can be determined.
• the "Activation time” indicates how quickly an alloy becomes “ready” for use as a hydrogen absorption alloy. Shorter activation times save energy and are indicative of fundamental material differences in the kinetic performance of the alloys. Note that activation is generally required only once in the life-cycle of a hydrogen storage alloy. Once the alloy has been activated, the hydrogen absorption time is significantly reduced as evidenced by the last cycles of the run.
• the magnesium nickel alloy of Example 1 was modified by the addition of a refining element.
• Table 1 shows the refining element and the addition rate of that element.
• activation time can be reduced to about 40% of that of the unmodified alloy (from ⁇ hours to 3.8hours). This is of practical significance but more importantly it is indicative of the superior kinetic performance of the modified alloy.
• Figure 6 is a summary of the maximum hydrogen absorption capacity taken from the results shown in Figure 5. It can be seen that at 350 0 C the maximum hydrogen storage capacity is similar (around 7 wt%) for all samples cast.
• the modified alloys are superior and the maximum hydrogen capacity can be increased more than 1 wt% relative to the unmodified alloy (from 5.3 wt% to 6.5 wt%).
• desorption temperature usually, at a fixed pressure, absorption temperature is lower than desorption temperature. The exact temperatures will vary depending on the pressure.
• Figure 7 shows the relationship between adsorption and desorption for the unmodified Mg 14Ni alloy at 0.2 MPa.
• the adsorption start temperature 1 is usually higher than the adsorption end temperature 2.
• the desorption start temperature 3 can be seen to be higher than the desorption end temperature 4.
• the desorption end temperature (plateau region of Figure 8) at 0.2 MPa decreases approximately 20 0 C with modification.
• the desorption temperature for the Mg Ni alloy can be reduced by trace element additions.
• the addition of the modifying elements increases the amount of internal interfacial areas within the material, the amount of stacking faults and the density of dislocations/twins in the solidified magnesium-nickel alloy. It is believed that the refining element should have an atomic radii in the range mentioned above in order to achieve the metallurgical effects in the as cast metal.
• the increase in dislocations caused by the additions is illustrated in the SEM micrographs Figures 9(a)-9(h).
• Figure 9(a) is the SEM for Mg 14 Ni unmodified; Fig 9(b) is the SEM for the same alloy with Zr addition; Figure 9(c) is the SEM for low sodium addition; Fig 9(d) is the SEM for high sodium addition; Figure 9(e) is the SEM for calcium addition; and Fig 9(f) is the SEM for Eu addition.
• Fig 9(g) is the Mg 14 Ni unmodified alloy of higher magnification; Fig 9(h) is the low sodium addition at the higher magnification of Fig 9(g).
• Figure 9 shows SEM secondary electron images of hypo-eutectic Mg-14wt%Ni alloys of

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• Battery Electrode And Active Subsutance (AREA)
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